In a storage system having controller units adopting a cluster structure (hereinafter abbreviated as CTL), a data cache or a disk pool and the like are shared among the respective CTLs, and a shared memory (hereinafter abbreviated as SM) is used to manage the same. The storage system prepares an area called a duplication area for storing a control information in the SM that must not be lost (hereinafter referred to as duplication information) and control information is stored thereto in duplicated manner so that the values of SMs arranged in two CTLs correspond. Other control information (hereinafter referred to as non-duplication information) are stored in the area called a non-duplication area.
Patent literature 1 discloses an art capable of accessing the SMs from all the micro-processors (hereinafter abbreviated as MPUs) by the same access cost (access performance such as access time). Patent literature 1 also discloses an art related to SM cache. The SM cache technique relates to caching a portion of the contents of the SM in a local memory (hereinafter abbreviated as LM) directly coupled to an MPU that can be accessed from the MPU more speedily than accessing the SM, according to which the SM access performance is enhanced. This technique enables to improve the access performance of the MPU to the SM. Patent literature 1 also discloses a method for controlling a cache memory (hereinafter abbreviated as CM) for temporarily storing data from the host computer or data from the drive unit, which differs from the SM or the LM.